The invention relates to propylene polymers. More particularly, the invention relates to a method for preparing propylene polymers which have increased molecular weight distribution.
Variables that have primary effect on the processing and physical properties of propylene polymers include molecular weight (normally expressed in terms of melt flow rates), molecular weight distribution (MWD), and stereospecificity. The weight average molecular weight (Mw) of propylene polymers ranges usually from about 200,000 to about 600,000. Molecular weight distribution is expressed as the ratio of Mw/Mn; Mn is the number average molecular weight. The ratio of Mw/Mn is also known as the polydispersity index (PDI).
The MWD has a significant effect on the processability and end-use properties of propylene polymers. Molten polymers are shear-sensitive, i.e., the apparent viscosity of molten polymers decreases as the applied pressure increases. Propylene polymers that have broad MWD are more shear-sensitive than those that have narrow MWD. As a result, broad MWD propylene polymers flow more readily in injection molding processes than narrow MWD propylene polymers.
The MWD of propylene polymers is a function of both the catalyst system and the polymerization process. Polymers produced with high-activity catalysts usually have narrow MWD. Therefore, propylene polymers produced with high-activity catalysts often need broadening their MWD. Post-treatments of polymers can increase MWD. However, post-treatments are often costly. More cost-effective methods involve modifying the polymerization process and catalyst systems so that the produced polymers have broadened MWD during polymerization.
For instance, U.S. Pat. No. 5,844,046 teaches the use of external donors such as silane compounds in the polymerization to broaden the MWD. However, this reference patent expressly teaches the use of a combination of at least two donors. According to this referenced patent, if only one silane compound is used, the produced polymers have relatively narrow MWD. The need for a combination of at least two donors complicates the polymerization process. Therefore, new methods for broadening the MWD of propylene polymers are needed. Ideally, the method would use high-activity Ziegler catalysts but use only one silane donor in the polymerization process.
The invention is a method for producing propylene polymers which have broad molecular weight distribution (MWD). The method comprises polymerizing propylene and an optional xcex1-olefin comonomer or an optional elastomer in the presence of a Ziegler catalyst and one silane donor. The silane donor is selected from vinyltrimethoxysilane or dicyclohexyldimethoxysilane. The resulting polymer has MWD greater than or equal to 7.0.
Contrary to the general belief in the art that a combination of two or more electron donors is usually needed (see, e.g., U.S. Pat. No. 5,844,046), we found that the use of one donor selected from vinyltrimethoxysilane or dicyclohexyldimethoxysilane is more effective in increasing the MWD of the polymers. For instance, when either vinyltrimethoxysilane or dicyclohexyldimeth oxysilane is used, the polymers produced have MWD greater than or equal to 7.0. We found that some other silanes, e.g., cyclohexylmethyldimethoxysilane, when used alone, are not effective to produce propylene polymers of broad MWD. We also found that although vinyltrimethoxysilane alone gives polymers of broad MWD, its combination with a second silane, e.g., cyclohexylmethyldi-methoxysilane, cannot produce broad MWD polymers.